This pokemon has nice special attack/special defense stats, but lacks defense, HP, and speed. I fear that this Pokemon can't properly defend itself, and makes an easy target for other Pokemon competitively.

Any advice?

Play Gardevoir to its strengths, like its high Special Attack and access to Trace. Otherwise, other Psychic Pokemon can do its jobs better (Jynx is faster and has access to Lovely Kiss, while Musharna and Reuniclus is bulkier, Alakazam is stronger and faster, etc.):
-Psychic
-Signal Beam
-Focus Blast
-Destiny Bond
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
Ability: Trace